1. Field
The present invention relates to a tire for a vehicle for agricultural use, such as an agricultural tractor or an agri-industrial vehicle.
It relates more particularly to the tread of such a tire, which is intended to come into contact with the ground via a tread surface.
2. Description of Related Art
In what follows, the circumferential, axial and radial directions respectively denote: a direction tangential to the tread surface of the tire and oriented in the direction of rotation of the tire; a direction parallel to the axis of rotation of the tire; and a direction perpendicular to the axis of rotation of the tire. The equatorial plane of the tire is the plane passing through the middle of the tread of the tire and perpendicular to the axis of rotation of the tire. An axial plane is a plane parallel to the axis of rotation of the tire: it is defined by the circumferential and axial directions.
A tire for an agricultural tractor is intended to be driven on various tires of ground such as the more or less compacted earth of the fields, unmetalled farm tracks providing access to the fields, and asphalt road surfaces. Bearing in mind the diversity of use, in the field and on road, a tire for an agricultural tractor and, in particular the tread thereof, has to exhibit a compromise in performance between traction in the field, resistance to chunking, resistance to wear on road, rolling resistance and on-road vibrational comfort.
In order to meet all these performance criteria, the tread of a tire for an agricultural tractor generally comprises a plurality of bars. The bars are elements that are raised relative to a base surface of the tread, of revolution about the axis of rotation of the tire, and extending radially as far as the tread surface.
A bar generally has an elongate parallelepipedal overall shape made up of at least one straight or curved portion, and is separated from the adjacent bars by furrows. A bar may be made up of a succession of straight portions, as described in documents U.S. Pat. No. 3,603,370, U.S. Pat. No. 4,383,567, EP 795 427 or may have a curved shape, as set out in documents U.S. Pat. No. 4,446,902, EP 903 249, EP 1 831 034.
In the radial direction, a bar extends from the base surface as far as the tread surface, the radial distance between the base surface and the tread surface defining the height of the bar. The radially outer face of the bar, belonging to the tread surface, which comes into contact with the ground as the bar passes through the contact patch, is referred to as the contact face of the bar.
In the axial direction, a bar extends inwards from an axial end of the tread. In general, a bar extends axially inwards from an axially outer end face as far as an axially inner end face closer to the equatorial plane of the tire.
In the circumferential direction, a bar extends, in a preferred direction of rotation of the tire, between a leading face and a trailing face. The preferred direction of rotation means the direction of rotation recommended by the manufacturer of the tire for optimum use of the tire. By way of example, in the case of a tread comprising two rows of bars in a V-shape or chevron configuration, the tire has a preferred direction of rotation according to the points of the chevrons. By definition, the leading face is the face the radially outer edge corner or leading edge corner of which is first to come into contact with the ground, as the bar passes through the contact patch, as the tire rotates. The trailing face is, by definition, the face the radially outer edge corner or trailing edge corner of which is last to come into contact with the ground, as the bar passes through the contact patch, as the tire rotates. According to the direction of rotation, the leading face is said to lead the trailing face.
The leading and trailing faces of a bar can be described in terms of the curves of intersection of the bar with an axial plane parallel to the axis of rotation of the tire and intersecting the bar. A bar thus, in any axial plane, comprises a leading profile, which is the intersection of the leading face and of the axial plane, and a trailing profile, which is the intersection of the trailing face and of the axial plane. The definition of the leading and trailing profiles, in any axial plane intersecting the bar and comprised between the base surface and the tread surface, defines the leading and trailing faces.
A bar usually has a mean angle of inclination with respect to the circumferential direction of close to 45°, the mean angle of inclination being the angle of the straight line passing through the axial ends of the bar. Specifically, this mean angle of inclination in particular allows a compromise between traction in the field and vibrational comfort. Traction in the field is all the better the more radial the bar is, i.e. the closer its mean angle of inclination is to 90°, whereas vibrational comfort is better the more longitudinal the bar is, i.e., the closer its mean angle of inclination is to 0°. It is a well-known fact that traction in the field is determined more by the angle of the bar in the shoulder region, and this has lead certain tire designers to propose a highly curved bar shape leading to a bar that is substantially radial at the shoulder and substantially longitudinal in the middle of the tread.
The tread of a tire for an agricultural tractor usually comprises two rows of bars as described hereinabove. This distribution of bars which are inclined with respect to the circumferential direction, gives the tread a V-shape commonly referred to as a chevron pattern. The two rows of bars exhibit symmetry with respect to the equatorial plane of the tire, usually with a circumferential offset between the two rows of bars, resulting from a rotation of one half of the tread about the axis of the tire with respect to the other half of the tread. Further, the bars may be continuous or discontinuous, and may be distributed circumferentially with a spacing that is constant or variable.
Various designs of bar tread have been proposed, depending on the desired improvement in performance, as shown by way of example by the documents mentioned hereinafter. Document U.S. Pat. No. 4,131,148 offers a base surface that is faceted to improve traction in the field and self-clearing of the tread. Document U.S. Pat. No. 4,611,647 offers a bar with a leading face of which the circumferential profile, in a circumferential plane parallel to the equatorial plane, is convex and curved, in order to improve resistance to wear, effectiveness and life. Document U.S. Pat. No. 5,010,935 describes a bar of which the leading face has a concave circumferential profile with a double slope, for better traction in the field and better resistance to the chunking. Document JP 11115417 describes a bar the leading face of which has a convex circumferential profile with a double slope to improve traction in the field and self-clearing of the tread.
However, conventional bar treads may be sensitive to a particular form of attack: attack by stubble remaining in the fields after harvest. Stubble is a portion of the stem of the plant the free end of which is sharp. The sharp free end of stubble, which comes into contact with the leading face of a bar, perforates it locally and superficially, leading to local chunking of the elastomer-based polymer or elastomer material of which the bar is made. Repeated attacks on the leading faces of the bars by stubble may cause severe deterioration to the appearance of the bars, or may even cause chunking particularly at the axially outer ends of the bars. This damage is likely to lead to complaints by the users and to the tire being replaced.